X-ray spectrometers are used to characterize the composition of object under investigation by analyzing the x-rays emitted from the interaction of a beam of ionizing radiation (e.g. electrons, protons, x-rays) with the object. Two commonly used spectrometers are the energy-dispersive and wavelength-dispersive types, the first of which directly detects x-rays using an energy-sensitive detector system, and the second of which uses a crystal, a synthetic multilayer, or diffraction device to direct specific wavelengths toward an x-ray detector.
Often there is need to analyze a small volume within the object. In general, systems incorporating such spectrometers typically rely on the focusing of the ionizing beam to achieve high lateral resolution (the direction perpendicular to the ionizing beam axis). But, in some cases, the achievable lateral resolution is larger the incident beam size. For example, in the case of electron excitation, the beam itself may be focused to less than 200 nm, but electron scattering in the object can generate fluorescent x-rays from an interaction volume on the scale of microns.
In some approaches, x-ray optics, polycapillary light guides or collimating apertures are between the object under examination and the spectrometer to spatially limit the region from which x-rays are detected. Such approaches are frequently used to achieve high longitudinal resolution (the direction along the direction of the ionizing radiation), which is especially important when the ionizing radiation consists of x-rays which penetrates deep in the object.
Several limitations exist with such approaches. Polycapillary optics, are non-imaging optics that do not provide point-to-point imaging of the focal spot; instead, they act analogously to “wave guides” that transport x-rays generated from the first focal spot onto the second focal spot in an incoherent manner. As a consequence, resolution is still inherently limited to the “focal” spot of the polycapillary. Furthermore, the polycapillary optics are not achromatic, so any spectra of x-rays will need additional calibration for the chromatic distortion. And, although optics with smaller focal spots may be used, the collection efficiency of such smaller optics drops significantly, resulting in weak signals and very long acquisition times. Additionally, absolute quantification of material composition is difficult or impossible due to the optics not being achromatic. Other approaches include, the use of apertures or small non-imaging monocapillary x-ray optics, but these dramatically reduce the signal detected.
There is a need for a method and a spectrometer system that can efficiently collect and spatially resolve the x-rays emerging from specific region(s) within an object under examination.